Devices such as a semiconductor laser diode (hereinafter referred to as “LD”) and a light emitting diode (hereinafter referred to as “LED”) are applied to a variety of apparatus such as optical disk devices. In recent years, in order to obtain such an optical disk device that is larger in storage capacity, LEDs and LDs are being developed using a nitride compound semiconducting material capable of emitting light in a blue to ultraviolet region that is short in wavelength. A nitride compound semiconductor has a high breakdown voltage owing to it's a large energy band gap. Also, being large in mobility and hence excelling in high frequency properties, a nitride compound semiconductor is being used to develop various types of transistors.
Nitride semiconductor devices such as LDs and high frequency transistors have thus been developed to form such a semiconductor device on a sapphire single crystal substrate, since a substrate of the nitride compound semiconductor such as gallium nitride (GaN) for example is not satisfactory in quality.
Conventional methods of making a LD using a nitride compound semiconductor was disclosed in JP H07-297495 A, which is referred to herein as a first prior art. FIG. 19 is a cross sectional view illustrating the structure of a LD according to the first prior art. As shown the LD, denoted by reference character 40, has a multilayer structure forming its active layers successively built on (11-20) planes of a single crystal sapphire substrate 41, the structure comprising an AlN buffer layer 42, an n+GaN layer 43, a nAl0.1Ga0.9N layer 44, a GaN layer 45 and a pAl0.1Ga0.9N layer 46 wherein the uppermost layer: p Al0.1Ga0.9N layer 46 has a SiO2 film 47 deposited thereon and opened for an electrode window where an electrode 48A is formed for the p layer 46. While an electrode 48B for the n layer 44 is formed on the n+GaN layer 43. The LD 40 has a Fabry-Perot resonator whose resonant faces are made by a pair of opposite end faces which are opposed to each other in a direction perpendicular to the optical path of laser light emitted (in a direction perpendicular to the sheet of paper).
The first prior art teaches that the opposed end faces defining the resonator are obtained by cleaving the sapphire single crystal substrate 41 along its <0001> axis (c-axis), followed by its splitting, and further teaches that this allows the efficiency of oscillations by the LD 40 using a nitride compound semiconductor to be improved by mirror-finishing the opposed end faces with precision.
On the other hand there is a report that a zirconium diboride (ZrB2) substrate is a promising substrate for a nitride compound semiconductor (J. Suda, H. Matsunami, Journal of Crystal Growth, Vol. 237-239, pp. 210-213, 2002), which is referred to herein as a second prior art. As taught there, a ZrB2 substrate can be obtained in the form of a single crystal in a floating zone method (FZ) by high frequency induction heating, is well lattice matched with a nitride compound semiconductor, and has a good electrical conductivity. It is further shown that a nitride compound semiconductor can be epitaxially grown on a ZrB2 substrate.
While the first prior art in making a LD seeks to provide a (10-12) cleavage face by cleaving the sapphire substrate 41 along its c-axis, the incongruity of such a crystallographic plane with a (10-10) plane in which the nitride compound semiconductor is to be cleaved gives rise to the problem that a stable cleavage face can seldom be obtained stably therefor. As a result, the problem in turn arises that because of the inability to ensure facial precision and parallelism of the opposed end faces providing the Fabry-Perot resonator for a nitride compound semiconductor, LDs high in oscillation efficiency cannot be obtained at an acceptable yield.
A LD using III-V group semiconductor is a so-called a vertical structure device in which the front surface of active layers and the rear surface of a substrate are provided with ohmic electrodes to flow the electric current from the front side to the rear side of the substrate. Such a vertical structure device cannot here be realized, however, since the sapphire single crystal substrate 41 being an insulator cannot provide an ohmic electrode at its rear surface. Thus, for the LD 40, it is necessary to form the electrode 48B on the n+ layer 43 as a lower part of the multi-layer structure and as on an upper surface of the device. Then, exposing the n+ layer 43 on an upper surface of the device in the course of crystal growth of LD multiple layers requires suspending crystal growth when the n+ layer 43 has grown and covering such an electrode area with an insulator such as SiO2 47 while preventing crystal growth, followed by crystal growth of further layers, and thus performing the so-called selective growth. And, after the multiple layers have been formed, it is required to perform a further step of exposing the n+ or electrode region to a device surface by etching. These added process steps give rise to problems of reduced yield and additional cost.
Further, the low thermal conductivity of a sapphire single crystal substrate 41 causes a nitride compound semiconductor such as a LD 40 using it to tend to rise in temperature while in operation with the result that the LD 40 has a very limited life and the high frequency transistor fails to achieve desired output and efficiency. Hence, there is the problem that a sapphire single crystal substrate 41 prevents a nitride compound semiconductor device from fully exhibiting its essential performance.
As for the second prior art, using a ZrB2 substrate as a substrate, for example, a LD requires the ZrB2 substrate to be cleaved simultaneously in forming the resonant faces of the LD. There is, however, the problem that the clear cleavage plane of a ZrB2 substrate has not yet been identified.